As a wireless power transmission system, an electric field coupling-type wireless power transmission system as disclosed in Patent Document 1 has been known, for example. In the system, power is transmitted to an active electrode of a power receiving device from an active electrode of a power transmission device with an electric field. In Patent Document 1, a switching frequency of a switching circuit that alternately drives a boosting circuit is swept in a state where a constant current is supplied to the switching circuit, and presence or absence of a maximum value in frequency characteristics of the voltage which is applied to the switching circuit is determined in the power transmission device. When the maximum value is present, the frequency is set as a driving frequency and power transmission is started. When the maximum value is absent, it is determined that the power receiving device is not mounted on the power transmission device and power transmission is not started.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2012-70614.
In Patent Document 1, the switching frequency of the switching circuit is swept so as to determine whether the power receiving device is mounted on the power transmission device. In this determination, it is necessary that the power transmission device keeps supplying a constant current to the switching circuit all the time. Therefore, in Patent Document 1, power consumption is increased in processing of detecting whether the power receiving device is mounted on the power transmission device. This raises a problem that power consumption in a standby state is increased.